This application claims benefit of Japanese Patent Application No. 2002-046324 filed on Feb. 22, 2002, the contents of which are incorporated by the reference.
The present invention relates to waveguide devices, in which optical fibers are optically connected to waveguide elements such as AWG (arrayed waveguide grating) and Mach-Zehnder interferometer and, more particularly, to waveguide devices adapted to reduce or eliminate optical coupling loss in such optical connection parts.
Recent rapid growth in the traffic of the Internet or telecommunication network systems demands high transmission speed and large capacity in the trunk transmission systems, called backbones. DWDM (Dense Wavelength Division Multiplexing) systems, in which multiple optical signals in different wavelengths are multiplexed, are useful to build large capacity transmission systems. The DWDM systems use waveguide devices such as AWGs for processing optical signals for multiplexing and demultiplexing optical signals. Such waveguide devices use optical fibers connected to waveguides for inputting and outputting optical signals.
In optical fiber communication systems, single-mode fibers and multi-mode fibers are used. Multi-mode fibers are large in the core diameter, and have a multitude of light beams. In single-mode fibers, only a single light beam (or two light beams when polarization is taken into consideration) is present. The respective modes have different propagation constants. In multi-mode fibers, the signal transmits in a multitude of different modes, which have different propagation constants, whereby the signal is distorted at the receiving side due to arrival time fluctuations. Single-mode fibers are free from multi-mode dispersion, and thus single-mode fibers are used in long distance transmission to avoid signal distortion by multi mode dispersion.
Single-mode waveguides, which have high coupling efficiency to single-mode fibers, are thought to become more and more important for future increases in the capacity and wavelength of optical communication systems. Such single-mode waveguides are important as structural elements of high performance multiplexers/demultiplexers, modulators, branches, couplers, interferometers, etc.
Heretofore, in single-mode waveguide design, the cross-sectional structure of a straight waveguide is determined to prohibit higher order mode. Bent waveguides, which can change the light beam direction, are necessary for realization of functional waveguide devices. The small bending radius of curvature causes small waveguide devices by requiring smaller area to change the light beam direction. However, the small bending radius of curvature causes unwanted radiation loss at the outside of the waveguide in the bend. This loss is called bending loss.
The AWGs and Mach-Zehnder interferometers are waveguide devices, which require such bent waveguides. Thus, for size reduction of the devices, it is necessary to make the radius of curvature of the bent waveguide as small as possible with taking the relation of the size reduction to the bending loss into consideration.
The minimum radius of curvature of the bent waveguide has a correlation to refractive index difference between core and clad of the waveguide. In other words, it is possible to reduce the minimum radius of curvature when larger refractive index difference can be used. To make the minimum radius of curvature to be 5 mm or less, the refractive index difference should be very large compared to the single-mode fibers and the waveguide core dimension should be very small compared to the single-mode fibers for preventing the multi-mode excitation.
However, the core dimension reduction of waveguides makes the spot size of the waveguides at the optical connection to the optical fibers smaller than the spot size of the single-mode fibers. This causes unwanted coupling loss in the optical connection.
In order to solve the problem in the case of larger refractive index difference of the waveguide compared to that of the optical fibers, it is necessary to make the spot size of the waveguides substantially equal to the spot size of single-mode fibers at the optical connection by gradually changing the core dimension of the waveguide. However, gradually changing the core dimension of the waveguide requires additional regions in the waveguide devices. This causes a new problem that the waveguide chip tends to be increased in size.
FIG. 6 illustrates this problem. The Figure shows a waveguide device 201, which comprises a waveguide chip 203 having a bent waveguide 202 and optical fibers 206 and 207 optically connected to two connecting parts 204 and 205 in the waveguide chip 203. The bent waveguide 202 is of single-mode, and its spot size is thus small compared to the case of assuming the multi-mode case. This means that the connecting parts 204 and 205, which are provided at the optical connection for making the spot size of the waveguide substantially equal to the spot size of the optical fibers, should have relatively large length dimensions a and b in the optical axis direction. In other words, at least the lengths a and b should be added to the length c of the bent waveguide 202 in this direction as the length of one side of the waveguide chip 203.